An electromagnetic flow meter is a measurement apparatus that includes a magnetic excitation coil for generating a magnetic field in a direction perpendicular to a flow direction of a fluid flowing in a measurement tube and a pair of electrodes disposed in the measurement tube and disposed in a direction orthogonal to the magnetic field generated by the magnetic excitation coil, and measures an electromotive force generated between the electrodes while alternately switching a polarity of an magnetic excitation current flowing to the magnetic excitation coil to measure a flow rate of a fluid to be detected flowing in the measurement tube.
In general, the electromagnetic flow meter is classified roughly into a contact type that measures the electromotive force of the fluid by directly contacting the electrode provided in the measurement tube with the fluid of a measurement target, and a capacitive type (non-contact type) that measures the electromotive force of the fluid through electrostatic capacitance between the fluid and the electrode without contacting the electrode provided in the measurement tube with the fluid of the measurement target.
The capacitive type electromagnetic flow meter calculates the flow rate by amplifying the electromotive force generated between the electrodes using a signal amplifying circuit (for example, differential amplifying circuit), then converting the amplified signal to a digital signal using an analog/digital conversion circuit, inputting the digital signal to a program processing device, such as a microcontroller, and executing predetermined arithmetic processing. In recent years, the capacitive type electromagnetic flow meter has particularly attracted attention since the electrode is difficult to deteriorate and maintenance is easy. As the related art of the capacitive type electromagnetic flow meter, for example, there are disclosures in PTL 1 and PTL 2.
Some electromagnetic flow meters have a function of measuring not only the flow rate of the fluid, but also electrical conductivity of the fluid (so-called conductivity). For example, PTL 3 discloses an electromagnetic flow meter including a two-electrode type electrical conductivity meter for obtaining the electrical conductivity by applying an AC signal, such as a sine wave or a square wave, between two electrodes and measuring a current flowing between the electrodes. In the electrical conductivity meter disclosed in PTL 3, two electrodes are immersed in a liquid of the measurement target at the same time to measure the electrical conductivity.